Classical homocystinuria due to enzyme defects in methionine or folate metabolism causes a 30 to 60 fold increase of plasma total homocysteine. The accumulation of homocysteine in these patients appears to be the primary factor for premature development of atherosclerotic changes and thromboembolism during childhood. Hence, it has been hypothesized that a less severe degree of homocysteinemia may be found in nonhomocystinuric subjects and may be associated with the occurrence of vascular disease in adult life. A positive correlation between plasma total homocysteine concentration and the occurrence of coronary artery disease, stroke and peripheral vascular disease has been observed in the past. However, the magnitude of homocysteine accumulation in these patients is mostly less than 2 fold of normal, contesting the premise that homocysteine is primarily responsible for the damage in vascular endothelium. To explore a potential direct relationship between homocysteinemia and vascular damage, it is desirable to investigate patients with intermediate levels of homocysteinemia. Recently, we have found a "new class" of homocysteinemia which is characterized by a 3 to 20 fold increase in plasma total homocysteine. These subjects have low normal or subnormal serum folate concentration, a partial deficiency and increased thermosensitivity of methylenetetrahydrofolate reductase. The goal of this proposal is to elucidate the biochemical and clinical nature of intermediate homocysteinemia and to develop treatment and prevention of homocysteine accumulation in these subjects. To achieve this goal, we will recruit intermediate homocysteinemics through screening subjects with subnormal or low normal levels of serum folate and patients with arteriosclerotic vascular disease. Since most homocysteine is present as the protein-bound form in nonhomocystinuric subjects and free homocysteine and cysteine-homocysteine mixed disulfide are readily bound to protein molecules, intermediate homocysteinemia will be identified by the determination of protein-bound homocysteine in stored specimens. Specific activity and thermosensitivity of methylenetetrahydrofolate reductase will be determined in lymphocyte extracts. Effects of folic acid therapy will be assessed biochemically and clinically. In addition, physiocochemical characterization of the defective enzyme will be carried out and antibodies against mutant enzyme will be prepared for a large scale screening of persistent intermediate homocysteinemia.